1. Field of the Invention
The present invention relates to wireless communication systems. More specifically, the present invention relates to the transmission and reception of physical downlink control channels and to the design of associated reference signals.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points, such as Base Stations (BS or NodeBs) to User Equipment (UE), and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals consist of data signals, carrying information content, control signals, and Reference Signals (RS), which are also known as pilot signals. A NodeB conveys data information to UEs through respective Physical Downlink Shared CHannels (PDSCHs) and control information through respective Downlink Control CHannels (CCHs). Multiple RS types may be supported, such as a Common RS (CRS) that can be used by all UEs and is transmitted over substantially an entire DL BandWidth (BW) and a DeModulation RS (DMRS) transmitted in a same BW as an associated PDSCH to a UE.
UL signals also consist of data signals, control signals and RS. UEs convey data information to a NodeB through respective Physical Uplink Shared CHannels (PUSCHs) and control information through respective Physical Uplink Control CHannels (PUCCHs). A UE transmitting data information may also convey control information through a PUSCH.
Downlink Control Information (DCI) serves several purposes and is conveyed through DCI formats in respective Physical Downlink Control CHannels (PDCCHs). For example, DCI includes DL Scheduling Assignments (SAs) for PDSCH receptions and UL SAs for PUSCH transmissions. The contents of a DCI format and consequently its size depend on the Transmission Mode (TM) a UE is configured for a respective PDSCH reception or PUSCH transmission. As PDCCHs are a major part of a total DL overhead, their required resources directly reduce DL throughput. One method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI). For Orthogonal Frequency Division Multiple (OFDM) as the DL transmission method, a Control Format Indicator (CFI) parameter transmitted through a Physical Control Format Indicator CHannel (PCFICH) can be used to indicate a number of OFDM symbols occupied by PDCCHs in a DL TTI.
FIG. 1 is a diagram illustrating a structure for a DL TTI according to the prior art.
Referring to FIG. 1, a DL TTI includes one subframe 110 which includes two slots 120 and a total of NsymbDL symbols for transmitting data information, DCI, or RS. OFDM is assumed for DL signal transmissions and an OFDM symbol includes a Cyclic Prefix (CP). A first MsymbDL symbols are used to transmit DL CCHs 130 and these MsymbDL symbols may be dynamically indicated in each DL TTI through a Physical PCFICH transmitted in a first subframe symbol (not shown). Remaining NsymbDL-MsymbDL symbols are used primarily to transmit PDSCHs 140. A transmission BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB includes NscRB sub-carriers, or Resource Elements (REs). A UE is MPDSCH allocated for a total of MscPDSCH=MPDSCH·NscRB REs for a PDSCH transmission BW. Some REs in some symbols contain CRS 150 (or DMRS), which enable channel estimation and coherent demodulation of information signals at a UE. A PDSCH transmission in a second slot may be at a same BW or at a different BW than in a first slot. In the former case, a PDSCH transmission is referred to as localized while in the latter case it is referred to as distributed.
Additional control channels may be transmitted in a DL control region but they are not shown for brevity. For example, assuming use of a Hybrid Automatic Repeat reQuest (HARQ) process for data transmission in a PUSCH, a NodeB may transmit HARQ-ACKnowledgement (ACK) information in a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether its previous transmission of each data Transport Block (TB) in a PUSCH was correctly detected (i.e., through an ACK) or incorrectly detected (i.e., through a Negative ACK (NACK)).
FIG. 2 is a diagram illustrating a DMRS structure according to the prior art.
Referring to FIG. 2, DMRS REs 210 and 215 in a RB over a subframe convey DMRS from four APs. A DMRS transmission from a first AP applies an Orthogonal Covering Code (OCC) of {1, 1} 220 over two DMRS REs located in a same frequency position and are successive in the time domain while a second AP applies an OCC of {1, −1} 225. A DMRS transmission from a third AP is in different REs than from a first AP and applies an OCC of {1, 1} 230 over two DMRS REs located in a same frequency position and are successive in the time domain while a fourth AP applies an OCC of {1, −1} 235. A UE receiver can estimate a channel experienced by a signal from an AP by removing a respective OCC at respective DMRS REs and also possibly by interpolating across respective DMRS REs.
FIG. 3 is a diagram illustrating an encoding process for a DCI format according to the prior art.
Referring to FIG. 3, a NodeB separately codes and transmits each DCI format in a respective PDCCH. A Radio Network Temporary Identifier (RNTI) for a UE, for which a DCI format is intended, masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable the UE to identify that a particular DCI format is intended for the UE. The CRC of (non-coded) DCI format bits 310 is computed using a CRC computation operation 320, and the CRC is then masked using an exclusive OR (XOR) operation 330 between CRC and RNTI bits 340. The XOR operation 330 is defined as: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC bits are appended to DCI format information bits using a CRC append operation 350, channel coding is performed using a channel coding operation 360 (e.g., an operation using a convolutional code), followed by rate matching operation 370 applied to allocated resources, and finally, an interleaving and a modulation 380 operation are performed, and the output control signal 390 is transmitted. In the present example, both a CRC and a RNTI include 16 bits.
FIG. 4 is a diagram illustrating a decoding process for a DCI format according to the prior art.
Referring to FIG. 4, a UE receiver performs the reverse operations of a NodeB transmitter to determine whether the UE has a DCI format assignment in a DL subframe. A received control signal 410 is demodulated and the resulting bits are de-interleaved at operation 420, a rate matching applied at a NodeB transmitter is restored through operation 430, and data is subsequently decoded at operation 440. After decoding the data, DCI format information bits 460 are obtained after extracting CRC bits 450, which are then de-masked 470 by applying the XOR operation with a UE RNTI 480. Finally, a UE performs a CRC test 490. If the CRC test passes, a UE determines that a DCI format corresponding to the received control signal 410 is valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards the presumed DCI format.
To avoid a PDCCH transmission to a UE that is blocking a PDCCH transmission to another UE, a location of each PDCCH in the time-frequency domain of a DL control region is not unique. Therefore, a UE must perform multiple decoding operations to determine whether there are PDCCHs intended for the UE in a DL subframe. The REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits in FIG. 2, a number of CCEs for a respective PDCCH depends on a channel coding rate (in the present example, a Quadrature Phase Shift Keying (QPSK) is used as the modulation scheme). A NodeB may use a lower channel coding rate (i.e., more CCEs) for transmitting PDCCHs to UEs experiencing a low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels may include, for example, of LCε{1,2,4,8} CCEs.
For a PDCCH decoding process, a UE may determine a search space for candidate PDCCHs after the UE restores the CCEs in the logical domain according to a common set of CCEs for all UEs (i.e., a Common Search Space (CSS)) and according to a UE-dedicated set of CCEs (i.e., a UE-Dedicated Search Space (UE-DSS)). A CSS may include the first C CCEs in the logical domain. A UE-DSS may be determined according to a pseudo-random function having UE-common parameters as inputs, such as the subframe number or the total number of CCEs in the subframe, and UE-specific parameters such as the RNTI. For example, for CCE aggregation levels LCε{1,2,4,8}, the CCEs corresponding to PDCCH candidate m are given by Equation (1).CCEs for PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is a total number of CCEs in subframe k, i=0, . . . , LC−1, m=0, . . . , MC(LC)−1, and MC(LC) is a number of PDCCH candidates to monitor in a search space. For example, for LCε{1,2,4,8}, MC(LC)={6,6,2,2}, respectively. For the CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1)mod D where Y−1=RNTI≠0, A=39827 and D=65537.
DCI formats conveying information to multiple UEs are transmitted in a CSS. Additionally, if enough CCEs remain after the transmission of DCI formats conveying information to multiple UEs, a CSS may also convey some UE-specific DCI formats for DL SAs or UL SAs. A UE-DSS exclusively conveys UE-specific DCI formats for DL SAs or UL SAs. For example, a UE-CSS may include 16 CCEs and support 2 DCI formats with L=8 CCEs, or 4 DCI formats with L=4 CCEs, or 1 DCI format with L=8 CCEs and 2 DCI formats with L=4 CCEs. The CCEs for a CSS are placed first in the logical domain (prior to interleaving).
The DL control region in FIG. 1 uses a maximum of M=3 OFDM symbols and transmits a control signal substantially over a total operating DL BW. As a consequence, such control region has limited a capacity and cannot achieve interference coordination in the frequency domain. There are several cases where expanded capacity or interference coordination in the frequency domain is needed for transmission of control signals. One such case is a communication system with cell aggregation where the DL SAs or UL SAs to UEs in multiple cells are transmitted in a single cell (for example, because other cells may convey only PDSCH). Another case is extensive use of multi-UE spatial multiplexing of PDSCHs where multiple DL SAs correspond to same PDSCH resources. Another case is when DL transmissions from a first NodeB experience strong interference from DL transmissions from a second NodeB and DL interference co-ordination in the frequency domain between the two NodeBs is needed.
A direct extension of the maximum DL control region size to more than MsymbDL=3 subframe symbols is not possible at least due to the requirement to support legacy UEs which cannot be aware of such extension. An alternative is to support DL control signaling in the conventional PDSCH region by using individual RBs to transmit control signals. A PDCCH transmitted in RBs of the conventional PDSCH region will be referred to as Enhanced PDCCH (EPDCCH).
FIG. 5 is a diagram illustrating an EPDCCH transmission structure according to the prior art.
Referring to FIG. 5, although EPDCCH transmissions start immediately after a conventional DL control 510 and are transmitted over all remaining DL subframe symbols, EPDCCH transmissions may instead start at a predetermined subframe symbol and extend over a part of remaining DL subframe symbols. EPDCCH transmissions may occur in four PRBs, 520, 530, 540, and 550, while remaining PRBs 560, 562, 564, 566, 568 may be used for PDSCH transmissions. As an EPDCCH transmission over a given number of subframe symbols may require fewer REs than the number of subframe symbols available in a PRB, multiple EPDCCHs may be multiplexed in a same PRB. The multiplexing can be in any combination of possible domains (i.e., time domain, frequency domain, or spatial domain) and, in a manner similar to a PDCCH, an EPDCCH includes at least one Enhanced CCE (ECCE).
A UE can be configured by higher layer signaling, such as Radio Resource Control (RRC) signaling, RBs that may convey transmissions of EPDCCHs. An EPDCCH transmission to a UE can be in a single RB if a NodeB has accurate DL channel information for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming. Otherwise, an EPDCCH transmission can be in multiple RBs. An EPDCCH transmitted over a single RB is referred to as localized while an EPDCCH transmitted over multiple RBs is referred to as distributed.
An exact design of a search space for EPDCCH candidates is not material to the present invention and may be assumed to follow the same or similar principles as a search space design for PDCCH candidates. Therefore, a number of EPDCCH candidates can exist for each possible ECCE aggregation level LE where, for example, LEε{1,2,4} ECCEs for localized EPDCCH and LEε{1,2,4,8} ECCEs for distributed EPDCCH. A UE determines EPDCCH candidates for each ECCE aggregation level in a search space according to predetermined functions similar to the one previously described for determining CPDCCH candidates for each CCE aggregation level.
The DMRS structure in FIG. 2 is intended for PDSCH transmissions and may not be appropriate for localized EPDCCH transmissions. In the following, two possible partitions of a RB over a subframe for constructing ECCEs are considered to illustrate some disadvantages of a DMRS structure as in FIG. 2 for localized EPDCCH transmissions.
FIG. 6 is a diagram illustrating a first option for an allocation of ECCEs in one RB over a number of subframe symbols according to the prior art.
Referring to FIG. 6, a partitioning of ECCEs is in the frequency domain, a RB includes 4 ECCEs, 610, 620, 630, and 640, and an EPDCCH transmission to a UE can consist of 1, 2, or 4 ECCEs. An EPDCCH transmission is assumed to start, for example, in a first subframe symbol after a conventional DL control region 650, if any, and extend in all remaining subframe symbols. Assuming that different beamforming can apply for EPDCCH transmissions to different UEs, a UE may only use a DMRS contained in ECCEs of a respective EPDCCH candidate. Then, as ECCE #2 does not contain any DMRS, it cannot be used for an EPDCCH transmission with an aggregation level of 1 ECCE. Moreover, as a DMRS is located in different REs in different ECCEs, a channel estimator should apply a different interpolation filter in the frequency domain depending on the ECCEs of an EPDCCH transmission. Therefore, a DMRS structure in FIG. 2 is inappropriate for localized EPDCCH transmissions using an ECCE partitioning as in FIG. 6.
FIG. 7 is a diagram illustrating a second option for an allocation of ECCEs in one RB over a number of subframe symbols according to the prior art.
Referring to FIG. 7, a partitioning of ECCEs is in the time domain, a RB contains 2 ECCEs, 710 and 720, and an EPDCCH transmission to a UE can consist of 1 or 2 CCEs. An EPDCCH transmission is assumed to start, for example, in a first subframe symbol after a conventional DL control region 730, if any, and continue in all remaining subframe symbols. As a DMRS transmission power in each of the 2 ECCEs can be different and depend on a transmission power required for a respective EPDCCH to meet a target reception reliability, it is not practically feasible to use a channel estimate that is derived from a DMRS in both ECCEs if a Quadrature Amplitude Modulation (QAM) is used to transmit an EPDCCH. Moreover, the 2 ECCEs are not equivalent as they may contain a different number of REs leading to different effective coding rates and different detection reliability for respective EPDCCHs. Also, an ECCE partitioning pattern may be variable depending on a number of subframe symbols for a conventional DL control region. Therefore, a DMRS structure as in FIG. 2 is inappropriate for localized EPDCCH transmissions with the ECCE partitioning as in FIG. 7.
In addition to increasing a capacity of DL control signaling and offering interference coordination in the frequency domain, another main design objective of EPDCCH is to improve a corresponding spectral efficiency relative to PDCCH, thereby reducing an associated overhead and improving DL throughput. In addition to conventional beamforming or FDS, other significant mechanisms for improving an EPDCCH spectral efficiency include the use of spatial multiplexing among EPDCCH transmissions and the use of QAM16 modulation. RBs configured for potential EPDCCH transmissions but not used to transmit any EPDCCH in a subframe should be available to a UE for PDSCH reception.
To facilitate spatial multiplexing of EPDCCH transmissions to different UEs, a DMRS AP associated with an EPDCCH transmission to a UE should be such that it allows a NodeB to flexibly apply spatial multiplexing of EPDCCH transmissions to two UEs, thereby effectively doubling a corresponding spectral efficiency.
In order to support EPDCCH transmissions with QAM16 modulation, in addition to QPSK modulation, without negatively impacting a system operation and a UE complexity, a use of QAM16 should avoid increasing (doubling) a number of EPDCCH decoding operations a UE needs to perform in order to avoid a respective increase in a UE receiver complexity and avoid increasing the probability of a false CRC check.
Therefore, there is a need to design a DMRS for localized EPDCCH transmissions.
There is another need for a UE to determine whether or not to include in a PDSCH reception a RB configured to the UE for potential EPDCCH transmission.
There is another need to increase a flexibility of a NodeB in applying spatial multiplexing for EPDCCH transmissions to different UEs.
Finally, there is another need to support QAM16, in addition to QPSK, for EPDCCH transmissions to a UE without increasing a number of decoding operations at the UE.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.